1. Technical Field of the Invention
This invention relates to a method for processing a thermoluminescent crystalline material, in particular a thermoluminescent radiation detector crystal. Still more particular, this invention relates to a method for processing irradiated crystalline material used in a thermoluminescence dosimeter badge. This invention also relates to an apparatus for processing such crystalline material.
2. Description of the Prior Art
It is well known to use radiation detector crystalline material such as calcium fluoride crystals or lithium fluoride crystals containing imperfections as radiation detector crystals in thermoluminescence dosimeter (TLD) badges. For instance, such badges are used to measure the exposure of the wearer thereof to ionizing radiation, such as gamma or beta radiation. Such radiation detector crystals are also used for environmental radiation monitoring, in particular for measuring low level radiation. After a certain exposure time, the radiation detector crystals are examined or read-out in order to determine the degree to which the wearer or the environment has been exposed to the radiation.
Thermoluminescence is the release of light in a crystalline material as it is heated at a predetermined rate. The light is a function of or even proportional to the previous nuclear radiation dosage which the crystalline material has received. Once the light is produced, the previous radiation history of the material is "erased" as the thermal cycling performs a destructive read-out of the sample's past history, see e.g. D. E. Lancaster "Thermoluminescence: Theory & Applications", Electronics World, March 1969.
Thermoluminescence can only be observed if the crystalline detector material has imperfections such as impurities locked in its lattice structures, or mechanical structure defects. These imperfections constitute electron traps. Popular dosimetry materials are, among others, calcium fluoride, calcium sulfate, lithium borate, lithium fluoride, and potassium sulfate (see: Electronics World, supra). The alkali halide lithium fluoride is frequently applied in dosimetry since it thermoluminesces brightly, is chemically stable, reasonably non-toxic, and easily formed into bar, rod, disk or powder. The impurities or activators which are purposely introduced into lithium fluoride in order to increase the number of electron traps are trace quantities of e.q. maqnesium, terbium, europium, or other rare-earth elements. A variety of TLD materials is described in the "TLD Materials Summary", September 1977, by The Harshaw Chemical Company, Solon, Ohio, U.S.A.
In the read-out step the crystals are heated at a constant rate, and a light vs. temperature curve called glow curve may be obtained. The glow curve has glow peaks that correspond to different energy-level traps.
While one should attempt to keep crystals clean, some dust or grease may become attached to them. It is important that this is removed before it is permanently burned into the surface (see e.g. Medical Physics Handbook No. 5, chapter "Thermoluminescence Dosimetry" by A. F. McKinlay (Adam Hilger, 1981), p. 138).
Before the actual read-out procedure, the crystals are usually annealed. It has been found for many TLD crystals that the anneal is necessary to reduce fading errors. For instance, for CaSO.sub.4 :Dy it has been found that due to the presence of low-temperature peaks in the glow curve, a post-irradiation anneal (T*=100.degree. C., t*=10 min) or a preheat incorporated into the reading cycle is definitely required; see review article "The Theoretical and Microdosimetric Basis of Thermoluminescence and Applications to Dosimetry", Phys. Med. Biol., 1981, Vol. 26, No. 4, 765-824, esp. p. 776. The effect of annealing has been thoroughly examined in the past (see e.g. B. Burgkhardt and E. Piesch, "The Effect of Post-Irradiation Annealing on the Fading Characteristic of Different Thermoluminescent Materials, Part II-Optimal Treatment and Recommendations", Nuclear Instruments and Methods 155 (1978), 299-304). The optimum annealing temperature T* and the optimum annealing time t* have been established for many materials, for instance for .sup.6 LiF:Mg dosed with thermal neutrons T*=60.degree. and t*=18 h (see R. B. Luersen and T. L. Johnson, "Optimum Over-night Annealing Temperature for LiF:Mg", presented at the 25th Annual Health Physics Society Meeting, Seattle, Wash., July 21-25, 1980), and for CaF and CaSO.sub.4 the temperature T* is 100.degree. C. and the time t* is between 10 and 25 minutes; (see B. Burghardt, D. Singh, E. Piesch, "High-Dose Characteristics of CaF.sub.2 and CaSO.sub.4 Thermoluminescent Dosimeters", Nuclear Instruments and Methods 141(1977), p. 363-368, in particular table 1.
At the present time, TLD crystals are conventionally cleaned with an organic solvent (see "Medical Physics Handbook, No. 5", supra). Then, in a separate step before read-out, they are annealed in an oven. In particular, lithium fluoride crystals are cleaned with warm trichloroethylene and/or a methanol rinse, and the clean crystals are subsequently heated in a 100.degree. C. oven for approximately 10 minutes (Communication of the Harshaw Chemical Company, Solon, Ohio, U.S.A.). The temperature of 100.degree. C. is the optimum annealing temperature of lithium fluoride crystals. Thereafter, the lithium fluoride crystals are read out.
It has been found that this two-step procedure of cleaning and annealing in a separate oven is time consuming and expensive.